1. The Field of the Invention
This invention relates to filters, and especially filter elements useful for filtering hot gases used in the deployment of passenger airbags, and to methods for making and using such filters, and to airbags and vehicles containing the same.
2. The State of the Art
Relatively recent concerns with passenger safety in land vehicles has led to the development of "airbag" technology, a passive restraint and protection system comprising a bag or pillow-like bladder that is inflated in an extremely short period of time using compressed or chemically-generated gas to fill the bag. The inflated bag is disposed or deployed between the front or side of the passenger and an interior portion of the vehicle's passenger compartment.
The first generation of pyrotechnic airbag vehicle occupant restraint systems used azide compositions (typically sodium azide, NaN.sub.3, mixed a heavy metal oxide) to generate the gas used to inflate the airbag. These explosive compositions generate a gas at over 1,000.degree. F. during the initial phase of the gas generation reaction. A large amount of condensable and molten and/or solid particulate matter is generated concurrently with the gas. Much of this matter is not only extremely hot but also of a caustic composition, and the particulate matter, travelling a high velocity, is potentially dangerous to the integrity of the bag and the occupant to be protected thereby. Some airbag designs included large vent holes in the bags for venting the gas into the passenger compartment, and so the gas used to inflate these bags must be filtered to prevent the particulates from entering the passenger compartment with the vented gas. In these designs, all of the gas generated escapes the reaction chamber and is propelled towards the airbag, so that the gases and any particulates would undoubtedly impinge at least on the bag itself if no filter were present. If no measures are taken to ameliorate the degradative effects of this mixed phase reaction mixture, the gases and/or particulates would penetrate the bag, likely causing its failure and, in the most serious situations, causing injuries to the passenger.
Various measures were taken to reduce the degradative effects of the gas, some of which are discussed in U.S. Pat. Nos. 4,902,036 and 5,318,323, both of which are incorporated herein by reference. One technique for reducing the degradative effects was the use of sacrificial layers to slow down the particulate material, and the use of static centrifugal or impingement particle separation techniques. The art also resorted to using denser and/or longer filter devices. With both of these approaches, there is a design trade-off between filtering the gases and providing a pressure drop small enough to avoid interfering with the rate at which the airbag inflates. There are other trade-offs, as particulate deflection devices are typically expensive machined parts which are fabricated from heavy steel plate (because they are not amenable to fabrication by stamping, their cost of manufacturing is increased). In general, the airbag designers had to contend with removing condensed solid poisonous products (usually unreacted sodium azide and sodium oxides produced in the reaction), cooling the gases before they inflated the cushions, and providing a homogeneous and uniformly distributed gas flow generated from an explosive source.
Many filtering devices used today comprise layers of metal screens of various mesh sizes and one or more layers of a non-combustible fibrous material packed between the screens. The efficiency of this type of filter is dependent upon how tightly the material is packed; a tighter packing leads to more efficient filtering but also to a higher pressure drop. According to the above-referenced '323 patent, there is also a problem with quality control in the mass fabrication of such screen-mat composites with respect to providing a uniform pressure drop across any given filter made.
Yet another problem in designing airbag filter devices is that as the filter becomes clogged, the pressure drop across the filter increases. Accordingly, the mechanical stresses on the filter are increased, and the gas and particulates move through the filter at a higher velocity, necessitating an improved filter strength and toughness to withstand the higher flow rate through, pressure drop across, and particulate velocity into the filter.
Besides the aforementioned patents, typical filters for airbags are made from a compressed wire mesh or steel wool, such as described in U.S. Pat. No. 3,985,076 (metallic mesh), EP 674,582 (sintered metallic fiber structure), U.S. Pat. No. 4,017,100 (multilayer structure of glass fibers, steel wool, and screens and perforated plates), DE 2,350,102 (glass wool), GB 2,046,125 (metal spheres partially sintered together to form a rigid, porous body), U.S. Pat. No. 5,204,068 (metal fiber felt comprising coated fibers, such as nickel, coated with silicon compounds), WO 94/14608 (metal wire mesh to which a non-woven web of metal fibers is bonded by sintering), and others, the disclosures of which are all incorporated herein by reference. The gas generating composition, often an azide (azoimide) composition with copper, generates hot gases and particles of copper slag. The desire of the designer is to filter the copper slag particles so that the molten metal droplets do not impinge the airbag. The final filter design became a trade-off between (i) having a sufficiently high density of filter material to catch the slag particles, (ii) providing sufficient mass in the filter to cool the filtered slag particles before they melt through the mesh or wool elements of the filter or fragment into smaller droplets that might do the same, and (iii) the total density and weight constraints of the filter. That is, if the filter is made of very fine wire mesh or wool to assure catching all of the molten slag particles, then the mesh or wool fibers will have insufficient mass to cool the impinged slag particle to a solid, and so the molten slag particle melts through the mesh or wool and/or it fragments into smaller particles that may eventually pass through the filter and impinge the airbag.
The new generation of gas generators employ cleaner and less toxic non-azide gas-generating compositions (e.g., as described in U.S. Pat. No. 5,525,170, disclosure of which is incorporated herein by reference) that provide relatively more gas than the azide-based compositions. While the need to filter the gas generated is thus less of a concern, federal government standards exist setting limits on the allowable amounts of soluble and insoluble particulates in the gas generated, and so there is still a need to filter the gas. The need to cool the gas generated is still a necessary step in the deployment of the airbag. Moreover, these newer generation gas generators still yield a significant explosive force against which the filter element must be stabilized.